Dark Energy
WFIRST's Primary Dark Energy Science Objective: Determine the expansion history of the Universe and the growth history of its largest structures in order to test possible explanations of its apparent accelerating expansion including Dark Energy and modifications to Einstein's gravity (Green et al 2011).
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Estimated distribution of dark matter and dark energy in the universe.
Image Credit: NASA
Revolutionary observations in the past two decades have shown that the expansion of the Universe is accelerating. Dark Energy is the proposed explanation for this acceleration. The nature of dark energy, however, remains a mystery. Alternately, the acceleration may be evidence of a breakdown in Einstein's theory of gravity on cosmic scales. In either case, the cause and history of cosmic acceleration is one of the most fundamental questions in physics today.
Dark Energy and the Universe
Gravity has the effect of slowing the expansion of the Universe, while dark energy has the opposite effect. It exerts negative pressure, accelerating the expansion. These competing effects also determine how structures of dark matter and of galaxies grow.
Image Credit: NASA/CXC/M.Weiss
The dark energy "equation of state" is expressed as:
Observations show that w is close to -1 at the present time.
If w is constant and equal to -1 the dark energy is described as a cosmological constant γ, first postulated by Einstein. WFIRST will help us to learn whether dark energy was constant over time or is more complex and has evolved.
The future value of w also tells us the future fate of the Universe. For -1 < w < -1/3, the Universe will undergo eternal expansion. Eventually other galaxies will disappear beyond the horizon, no longer visible as the speed of light is insufficient to reach us as the Universe expands. For w < -1, the dark energy density will become infinite. Even gravitationally bound structures would be torn apart.
Measuring Dark Energy
In order to understand the nature of dark energy, WFIRST will measure the expansion history of the Universe and the growth of large-scale structure (the clustering of galaxies and their associated halos of dark matter in the Universe). It will use three complementary techniques:
The 2010 National Research Council New Worlds, New Horizon Decadal Survey of Astronomy and Astrophysics report stated: "Why should WFIRST employ all three methods? Supernovae (in particular, type SNe Ia) give the best measurements of cosmic acceleration parameters at low redshift due to their greater precision per sample or per object. BAO excels over large volumes at higher redshift. Together SNe Ia and BAO provide the most precise measurements of the expansion history for 0 < z < 2 and place significant constraints on the equation of state. Weak-lensing provides a complementary measurement through the growth of structure. Comparing weak-lensing results with those from supernovae and BAO could indicate that cosmic acceleration is actually a manifestation of a scale-dependent failure of general relativity. Combining all three tests provides the greatest leverage on cosmic acceleration questions. WFIRST can do all three."
Standard Candles and Standard Rulers
Image Credit: NASA/JPL-Caltech/R. Hurt (SSC)
In cosmology, we derive distances from "standard candles" - objects of known intrinsic brightness. We observe how bright the object appears to be to us, and comparing that to the intrinsic brightness, we derive the distance. We can also understand the expansion history using "standard rulers" - known intrinsic separation between objects.
Dark Matter
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Image Credit: NASA,ESA, Caltech; R. Massey
Dark matter comprises the bulk of the mass of the Universe, yet is notoriously difficult to study. This is due to the fact that dark matter neither absorbs or emits light (thus earning the moniker "dark"). Dark matter is therefore detected by its gravitational influence on luminous sources such as galaxies. In the past, evidence for dark matter has come from measuring the rotational curves of galaxies and from measuring the relative velocities of galaxies in clusters. In both cases, it is apparent that there is much more mass than what we can see in the form of stars, dust, and other forms of "normal" matter. More recently, even more conclusive evidence for dark matter has come from gravitational lensing, the distortion of the apparent shapes of background objects by foreground dark matter; this gravitational bending of light is a direct consequence of Einstein's General Theory of Relativity.
Supernovae 1a
The Type Ia SN 1994ae, discovered by S. Van Dyk and the Leuschner Observatory Supernova Search (IAU Circular 6105) on Nov. 14, 1994, in the nearby spiral galaxy NGC 3370. This image is from the 1.2-meter telescope at the Fred Lawrence Whipple Observatory and was obtained in good conditions a few weeks after maximum light. The supernova peaked at ~13th magnitude in the visual. Extensive monitoring of the light curve in 5 colors was obtained beginning 2 weeks before maximum and provides one of the most complete photometric records of a supernova light curve
Image Credit: NASA and A. Riess (STScI)
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Joint confidence intervals derived from SN samples for a two-parameter model of the equation-of-state parameter of dark energy, w(z) = w_a + w_a*(z/(1+z)), where z is redshift. For each panel, constraints from a SN sample are combined with the indicated prior to yield the indicated confidence intervals. The position of a cosmological constant is shown as a filled symbol.
Image Credit: Riess et al. (2007, ApJ, 659, 98; Copyright 2007, the American Astronomical Society
Weak Gravitational Lensing
Image Credit: Jason Rhodes (JPL)
The light from distant galaxies travels through dark matter structures along its journey to our telescopes. As a consequence of General Relativity, the path of the light is distorted by the gravitational influence of the dark matter, resulting in distortions to the observed shapes of these galaxies. By looking for small, coherent distortions of galaxies shapes, we can piece together the dark matter distribution along the line of sight to these distance galaxies. The evolution of these dark matter structures over cosmic time is driven by an interplay between the attractive force of gravity and the repulsive dark matter. Thus, by observing the growth of these dark matter structures at a range of distances (corresponding to a range in time) we can understand dark energy and gravity.
Baryonic Acoustic Oscillations
Large-scale redshift-space correlation function of the SDSS LRG sample, plotting the correlation function times s^2. The models are Omega_m h^2 = 0.12 (green line), 0.13 (red line), and 0.14 (blue line), all with Omega_b h^2 = 0.024 and n = 0.98 and with a mild nonlinear prescription folded in. The magenta line shows a pure CDM model (Omega_m h^2 = 0.105), which lacks the acoustic peak.
Image Credit: Eisenstein et al., 2005 ApJ, 633, Copyright 2005, the American Astronomical Society.
The early Universe was much hotter and denser than the Universe we see today. There were no stable atoms, only a hot plasma of electrons, protons, photons, and dark matter. Sounds waves could travel through this plasma, like the ripples on a pond. The speed of sound, then, determined a characteristic scale.
By 370,000 years after the Big Bang, the Universe had expanded and cooled sufficiently that photons no longer had enough energy to break apart Hydrogen atoms. This change, called "recombination", fixed the distance scale imposed by the sound speed in the plasma before. Small fluctuations in the matter density (1 part in 10,000) grew into the large structures in the Universe.
At that time, the Universe became transparent, and the photons could now travel unimpeded but shifted to longer wavelengths by the expansion of the universe. These photons today are the Cosmic Microwave Background. Measurement by WMAP and soon Planck provide a precise determination of the characteristic scale in the Early Universe.
The distribution of galaxies allows us to measure this characteristic scale at later epochs, and serves as a "standard ruler". The technique of using the small fluctuations in the early Universe, as determined by the sound speed, as a ruler is known as Baryonic Acoustic Oscillations.
The Sloan Digital Sky Survey provided spectacular confirmation of this prediction. By measuring the galaxy correlation function for ~50,000 galaxies, SDSS determined the most likely distance between galaxies, which results from the BAO, is the equivalent of 480 million light years today. This result confirmed the basic picture of structure formation through gravity, and the necessity of dark energy (without it the scale would have been smaller).
